Louisville Medicine Volume 69, Issue 2 - Page 25

of gun violence plaguing this country . Another important cause is iatrogenic injury related to positioning , regional anesthesia and surgical procedures , especially at the shoulder .
The clinical features of brachial plexus trauma depend upon the part of the plexus that has sustained the injury . It may affect the upper plexus ( Erb ’ s palsy ) or the lower plexus ( Klumpke ’ s palsy ) or the entire plexus . From the surgical point of view , it is crucial to know if the injury is at the nerve root level ( root avulsion ), which is proximal to the dorsal root ganglion ( preganglionic injury ), carries a poor prognosis for spontaneous recovery and tends to be non-operative . In such lesions , since the cell body of the sensory neuron is in continuity with the nerve , although the patient has no perception of sensations , paradoxically the sensory nerve action potential ( SNAP ) is intact on nerve conduction studies . On the other hand , in an injury distal to the dorsal root ganglion ( postganglionic ), the axons in the nerve are disconnected from the cell body and hence the SNAPS are absent . Root avulsions can be further substantiated by the use of sensory evoked potentials and by imaging studies .
There are different grades of injuries with differing prognosis . Injuries to the nerves can cause loss of the insulating myelin sheath ( demyelination ) leading to conduction block ( neuropraxia ) or disruption of axonal continuity ( axonotmesis ) or severance and discontinuity of the nerve ( neurotmesis ). Both axonal and nerve discontinuity set in motion a cascade of events resulting in Wallerian degeneration ( named after Augustus Waller ) 2 and eventually denervation atrophy of muscles . Reinnervation of muscles may occur by a process of axonal regrowth ( 1mm / day ) in cases of complete injury or by axon terminal sprouting in partial injury . The existing intact axon terminal tends to reinnervate the denervated muscle fibers ( much faster than axonal regrowth ) leading to quicker restoration of function . The traditional surgical repair consisted of reconnecting the severed ends in cases of neurotmesis or use of nerve grafts from sural nerve of the patient or by the use of collagen tubes . These procedures took long hours and often the results were not impressive except in the very young .
Let us see what is in the horizon in the field of nerve repair . There are three areas of translational research that hold promise .
Since Wallerian degeneration is the underlying process after axonal injury , leading to denervation atrophy of muscles , can it be postponed or avoided ? Recent discovery of genetic mutations that delay Wallerian degeneration ( slow Wallerian degeneration in mice in which transected axons do not degenerate for weeks 3 ) has provided unique insight into the process and holds promise for significant advances in treatment . It appears that a chemical , nicotinamide mononucleotide adenyl transferase 2 ( NMAT2 ) is essential for axon growth and survival ; loss of it may be the trigger for Wallerian degeneration .
Unlike the central nervous system , the peripheral nervous system possesses intrinsic capability to regenerate , as axons can regrow over long distances to reach their final target and Schwann cells are able to remyelinate them . But the rate of growth is slow . Can we hasten this process and also avoid misdirection of axonal growth ? An interesting observation is that brief low frequency electric stimulation has been found to be capable of accelerating axonal regrowth . 4 This is a topic of active research and may provide future guidelines for intra and postoperative electric stimulation during nerve repair .
There is growing enthusiasm about the use of nerve transfers to provide quicker reinnervation of denervated muscles . The slow regrowth of axons often leads to unsuccessful reinnervation as the muscle may be fibrotic by the time the regenerating axons reach the target . Transfer of an undamaged nerve to the injured nerve close the motor end plate zone may lead to fast reinnervation . This was initially popularized by Oberlin 5 who used a branch of ulnar nerve to connect to the musculocutaneous nerve to quickly reinnervate the biceps in upper plexus injuries including C6 root avulsion . The success paved way to the same technique being used in many other situations : use of a branch of radial nerve to innervate deltoid via axillary nerve ; terminal branch of anterior interosseous nerve to reinnervate ulnar nerve-innervated intrinsic hand muscles .
There is hope . What nature has been hiding within the injured nerve , now we can see by combining electrical studies with ultrasound and magnetic resonance neurography and furthermore , recent advances in surgical techniques are promising better outcomes .
“ There is no medicine like hope , no incentive so great , and no tonic so powerful as expectation of something tomorrow .” ( Orison Swett Marden ) References
Andersen J et al . Perinatal brachial plexus palsy . Paediatr Child Health . 2006 ; 11 ( 2 ): 93-100
Griffin JW . Augustus Waller and the case of the disappearing axon . Nat Rev Neurol Neurology 200 ; 7 ( 3 ): 355
Conforti L et al : Wallerian degeneration : an emerging axon death pathway linking injury and disease . Nature Reviews Neuroscience . 2014:15 : 394-409
Gordon T . Electrical stimulation to enhance axon regeneration after peripheral nerve injuries in animal models and humans . Neurotherapeutics . 2016 ; 13 : 295-310
Oberlin C et al . Nerve transfer to biceps muscle using a part of ulnar nerve for C5-C6 avulsion of the brachial plexus . J Hand Surg . 1994 ; 19:232-237
Dr . Iyer practices at the Neurodiagnostic Center of Louisville and is a retired professor of neurology at the University of Louisville School of Medicine .
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